We have shown that cell division in the amphibian lens epithelium depends upon pituitary hormones. The object of the proposed work is to define the mechanism by means of which hormonal control is achieved. It is probably not a primary pituitary hormone but a trophic factor(s), produced in the presence of one or more of these hormones, that directly promotes cell division. We want to determine the time at which the trophic factor(s) is produced and also its chemical nature. Experiments will be pursued in vivo and in organ, as well as tissue culture systems. Techniques to be employed will be gel electrophoresis, autoradiography, microspectrophotometry and microfluorometry. Since cell division is completely stopped after hypophysectomy, we will be able to study the contribution of proliferation to lens morphogenesis. No other in vivo system has yet been reported (in a vertebrate organism) where mitosis is completely controlled by factors produced at a known anatomical site.